Extreme heat will shift westward later in the week, with cooler air arriving in the Midwest and much of the East.  However, above-normal temperatures will persist across the Deep South. 

During the weekend, triple-digit (100-degree) heat will affect the Plains, extending as far north as the Dakotas.  Summer-like heat will also stretch from the Desert Southwest to the Rockies, although cool weather will prevail along and near the Pacific Coast.

Around the periphery of a ridge of high pressure, scattered showers and thunderstorms will affect the northern and eastern U.S. 

…